Re: [HACKERS] PostgreSQL - Weak DH group

On 07/13/2017 10:13 PM, Robert Haas wrote:
> On Thu, Jul 13, 2017 at 1:30 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Heikki Linnakangas <hlinnaka@iki.fi> writes:
>>> I don't think this can be backpatched. It changes the default DH
>>> parameters from 1024 bits to 2048 bits. That's a good thing for
>>> security, but older clients might not support it, and would refuse to
>>> connect or would fall back to something less secure.
>>
>> Do we have any hard information about which versions of which clients
>> might not support that?  (In particular I'm wondering if any still exist
>> in the wild.)
>
> Yeah.  If we break clients for v10 two months from release, some
> drivers won't be updated by release time, and that sounds pretty
> unfriendly to me.  On the other hand, if there is only a theoretical
> risk of breakage and no clients that we actually know about will have
> a problem with it, then the argument for waiting is weaker.  I'm not
> generally very excited about changing things after beta2, which is
> where are, but if this is a security issue then we might need to hold
> our nose and go ahead.  I'm against it if it's likely to cause
> real-world connectivity problems, though.

Googling around, I believe Java 6 is the only straggler [1]. So we would 
be breaking that. Java 7 also doesn't support DH parameters > 1024 bits, 
but it supports ECDHE, which is prioritized over DH ciphers, so it 
doesn't matter.

Java 6 was released back in 2006. The last public release was in 2013. 
It wouldn't surprise me to still see it bundled with random proprietary 
software packages, though. The official PostgreSQL JDBC driver still 
supports it, but there has been discussion recently on dropping support 
for it, and even for Java 7. [2]

I would be OK with breaking DH with Java 6 in PostgreSQL 10, especially 
since there's a simple workaround (generate a 1024-bit DH parameters 
file). I would be less enthusiastic about doing that in a minor release, 
although maybe that wouldn't be too bad either, if we put a prominent 
notice with the workaround in the release notes.

[1] https://wiki.mozilla.org/Security/Server_Side_TLS#DHE_and_ECDHE_support

[2] 
https://www.postgresql.org/message-id/69ae857b-15cc-36dd-f380-6620ef1effb9%408kdata.com

- Heikki